1. Field of the Invention
The present invention is in the field of ion chromatography and specifically relates to a technique for measuring concentration of ions in the parts per trillion range (10.sup.-5 to 10.sup.-6 milligram per liter).
2. The Prior Art
The cost of shutting down a large power plant for repairs or maintenance may be as much as $140,000 per hour. Clearly, it is in the interest of the owner to take such measures as may be available to increase the length of time between such occurrences.
Corrosion of the pipes in a steam-generating installation is a major concern. Corrosion weakens the pipes and could lead to a dangerous situation if not controlled. The high pressures and temperatures used in contemporary steam plants greatly accelerates the corrosion process and thereby shortens the life of the installation.
In an effort to reduce corrosion, power plant operators have continually attempted to improve the purity of the water used. The presence of certain ions in the water is especially undesirable, and a substantial effort is made to reduce the concentration of those ions.
In order to reduce the concentration of harmful ions in the water, it is necessary to be able to measure their concentration with satisfactory accuracy. Typically, the concentration of those ions is in the range 10.sup.-4 to 10.sup.-6 mg/1. Considerable difficulty is encountered in measuring such minute concentrations. The instrument most commonly used is an ion chromatograph. As normally used, a prepared sample of accurately known concentration and volume is conducted through the chromatograph, and it produces a reading that is measured. Thereafter, a sample of unknown concentration is also run through the chromatograph, and the magnitude of its reading is compared to the magnitude of the reading produced by the sample of known concentration. The linearity of the instrument being well-known, the concentrations are in proportion to the readings produced.
This procedure is quite acceptable so long as a sample of known concentration, also called the standard sample, is available. However, it is very difficult to obtain and maintain samples in the parts per trillion range. At such extraordinarily low concentrations, the sample readily becomes contaminated, not only from the vessels that contain it, but also from any air that may come in contact with the sample. In practice, concentrations on the order of parts per billion (1O.sup.-3 mg/1) represent the practical limit for field use and for use in automatic measuring equipment.
In addition to the possibilities of contamination, it is not practical to obtain a parts per trillion standard merely by mixing a small amount of parts per billion standard with a large quantity of water. The water itself is not sufficiently pure.
This lack of a practical concentration standard in the parts per trillion range has impeded efforts to measure concentrations in that range by use of the ion chromatograph.
At these concentrations, there was little prior art to guide the present inventor, who has devised his own solution to the problem.
The method used by the present inventor is believed to be novel, notwithstanding the fact that some elements of the apparatus used by the present inventor are known in the art.
For example, in U.S. Pat. No. 3,559,458 issued Feb. 2, 1971, Hrdina states that it is known to insert a sample from a capillary tube into a buffer stream by the use of manually-operated valves. He also states that the use of a special multiway valve to connect a loop containing a sample to a gas stream circuit is known in gaseous chromatography.
In U.S. Pat. No. 3,975,946 issued Aug. 24, 1976, Ball, et al. describe a sample introduction apparatus that includes a valve body that has at least one fluid passage of precisely defined volume, which receives and holds the sample. Thereafter, the valve body is rotated to permit a carrier liquid to positively displace the precisely defined volume of sample fluid into the chromatograph.
In U.S. Pat. No. 4,102,782, issued July 25, 1978 Saito, et al. teach the use of a pulse motor to drive a syringe for the purpose of accurate sample introduction.
U.S. Pat. No. 4,715,216 issued Dec. 29, 1987 to Muller describes apparatus and method for the determination of low ion concentrations in aqueous specimens by ion chromatography at the parts per billion level. This patent typifies the state-of-the-art at the time of the present invention.